Here is a link to a free site that checks all your medications for drug interactions. It is really good. A word of caution: you may find that it tells you there is a possible conflict in some of medications. This can be good because it did alert me to a conflict that the doctor didn't catch. I also found that with the number of meds I am taking it was important to take a few notes.
However, most of the time the interaction may be mild enough that it is still ok to take the med. Ideally you could ask your doctor - I would have to wait a long time for an answer from mine.
Cold medications can make you feel better and all of the packaging cautions you about use if you are on blood pressure meds and certain psychiatric meds.
Vitamin C drops should be ok.
Bad colds going around - unfortunately Lyme treatment doesn't protect us. I had a horrible one at Thanksgiving.